The United States and Japan will step up their defence cooperation to deal with the threat from nuclear-armed North Korea as tensions in East Asia remain high, officials from the two allies said on Thursday.

Father and son attacked 91-year-old woman

A 91-year-old woman who was bashed and robbed by a father and son burglary team in her Melbourne home was threatened with rape if she went to police, court documents reveal.

Gino Kovac, 49, and his son, Michael, 29, both pleaded guilty in the Melbourne Magistrates Court on Thursday to charges including recklessly causing serious injury and burglary, stemming from the September 17 attack in the city's southeastern suburbs.

The men broke in through the front window of the victim's Mount Waverley home, tackled her to the ground, hit her and dragged her from room to room, demanding money and jewellery, the court heard.

The woman spent a week in hospital, suffering bleeding on the brain, cuts to the head and extensive bruising.

At one point, Gino Kovac grabbed her around the neck and began to strangle her out of frustration at being unable to find where her jewellery was kept, the woman said in a statement tendered in court.

"I was scared and did what they said because I was fearful that if I didn't and they got angry then they would finish me off and kill me," she said.

Before the pair fled with cash and jewellery, Michael Kovac warned the victim as she lay bleeding on the floor, "If you go to the police I'll come back and rape you."